Along with the recent widespread of electrophotographic printing equipment and laser printers, a direct drawing type lithographic printing plate material, with which a printing plate can be directly made by such equipment, is widely used in the light printing field.
Such a direct drawing type lithographic printing plate material adopts a structure that an image-forming layer having suitability to a humidifying solution (fountain solution) in offset printing is provided on a support such as paper or a plastic film.
In this image-forming layer, image portions formed by attaching a toner become lipophilic and carry a printing ink, while non-image portions become hydrophilic, carry a humidifying solution and repel the printing ink, thus enabling offset printing.
Therefore, the image-forming layer preferably has sufficient water retention property.
Meanwhile, when images are outputted to the direct drawing type lithographic printing plate material by an electrophotographic laser printer, scattering of the minute toner onto the non-image portions cannot be avoided, and an offset ink is attached to these portions upon printing and reproduced on the printed matter as printing scumming. In an actual printing work, such printing scumming can be prevented by increasing the amount of the humidifying solution to be supplied. However, since water retention property is not sufficient, that is a so-called “water allowance”, which is a tolerance of the humidifying solution, is narrow in a conventional direct drawing type lithographic printing plate material, a problem arises that so-called “overemulsification” such as lower printing density or image blur occurs when the amount of the humidifying solution to be supplied is increased.
Further, when a printing machine is once stopped, the surface of the plate material becomes dry. Therefore, at an initial stage after an operation of the printing machine is resumed, a printing ink is also impressed onto non-image portions. In this case, since water retention property is inferior in the conventional direct drawing type lithographic printing plate material, once impressed ink is hardly removed. Thus, it takes a long time from the time when supply of the humidifying solution is started to the time when contamination of a plate material is removed and, as a result, a large amount of defective printed matter is produced. Further, when the contamination is not removed, the plate needs to be replaced.